Fracture toughness of PMMA as measured with indentation cracks
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J. A. Salem NASA Lewis Research Center, Cleveland, Ohio 44135 (Received 28 December 1992; accepted 10 August 1993)
Fracture toughness of polymethyl methacrylate (PMMA) was evaluated with indentation cracks. The cracks were developed by indenting in a liquid acetone environment, as suggested by previous researchers. Due to the inconsistency in crack configurations with varying indentation loads and to the negligible residual stress from indentation, the application of the indentation strength method was limited in evaluating fracture toughness of PMMA. The semielliptical crack approximation, however, particularly at a low indentation load of 9.8 N, resulted in reasonable agreement with the value determined by three conventional fracture toughness testings using the compact tension (CT), double cantilever beam (DCB), and single edge notched beam (SENB) specimens. Measurements at other indentation load typically were in poor agreement with conventional methods due to poorly developed crack configurations.
I. INTRODUCTION The use of indentation cracks is well established for studying the mechanical properties of brittle materials, such as glasses and ceramics. The attraction of indentation techniques is their simplicity as a means of introducing a small, predetermined flaw configuration into test samples. Particularly, indentation techniques have been used to determine the fracture toughness of brittle ceramic materials for more than a decade 1 ^ and more recently to evaluate fracture toughness of a brittle intermetallic material,5 although the underlying analysis is not theoretically based in a rigorous sense because of the complexities associated with residual stresses produced by the elastic/plastic deformation of the indentation.1"4 Ritter et al.6 studied the applicability of indentation techniques in order to characterize the failure behavior of PMMA. They succeeded in producing cracks in the sample surface by using a Vickers microhardness indenter in the presence of liquid acetone. They observed that the cracks formed by indentation with loads from 100 to 1400 N had palmqvist configuration where the ratio of crack depth to radial crack size was about 0.5. Based on the indentation strength data determined as a function of indentation load, they obtained a fracture resistance curve (R-curve) in which fracture toughness increased from 1.42 to 2.05 MPaVm with corresponding indentation loads from 100 to 1400 N. 7 In their calculation of fracture resistance, they assumed that the crack shape
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NASA Resident Research Associate, Lewis Research Center, Cleveland, Ohio 44135.
3210 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 12, Dec 1993 Downloaded: 11 Mar 2015
was better approximated as a semiellipse and that the critical crack dimension with respect to strength was the crack depth (minor axis) with a constant minor- to majoraxis ratio of 0.5. However, they did not provide any rigorous comparison of fracture resistance obtained by the indentation technique to more conventional fracture toughness testi
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